Particle compositions have varied uses and are ubiquitous in applications that exploit surface chemistry and physics. As the mean particle size of the composition is reduced, the surface area increases with the square of the particle size. This results in a corresponding increase in surface functionalities, e.g., reaction rate, due to the increase in available surface area. Examples of systems that rely on high surface area for optimal performance include catalytic converters, dye sensitized solar cells, batteries, and fuel cells. Some of these applications use nanoparticle films that consist of more than one type of nanoparticle. In such a system, various particles perform different functions.
In certain types of fuel cells, for example, the simultaneous transport of protons and electrons requires some components of the film to serve as electron conductors, and others to serve as proton conductors (Haile, Chisoholm, Sasaki, Boysen, and Uda, “Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes”, Faraday Discussions 134: 17-39, 2006). In another example, a film of at least two different sizes of titania nanoparticles may be used to optimize the performance of a dye sensitized solar cell (Vargas, “Aggregation and composition effects on absorption and scattering properties of dye sensitized anatase TiO2 particle clusters”, Journal of Quantitative Spectroscopy & Radiative Transfer 109: 1693-1704, 2008). In such a system, the larger particles can scatter more of the incident light for more efficient collection by the smaller particles, which dominate the surface area and thus photon-induced electron excitations.
A number of methods for depositing a particle film are known where the particles are of the same composition. Tolmachoff et al. (Tolmachoff, Garcia, Phares, Campbell, and Wang, “Flame synthesis of nanophase TiO2 crystalline films”, Proceedings of the 5th U.S. Combustion Meeting, Paper #H15, 2007) discloses a method for making thin films of titania particles by repeatedly passing a substrate over a stagnation flame. Other methods of making single component nanoparticle films include screen-printing or squeegeeing a nanoparticle paste (Llobet et al., “Screen-printed nanoparticle tin oxide films for high-yield sensor microsystems”, Sensors & Actuators B 96: 94-104, 2003), printing a micro- or nano-particle ink (US patent applications 20070169812 and 20070169813), chemical vapor deposition (Zhu et al., “Growth of high-density Si nanoparticles on Si3N4 and SiO2 thin films by hot-wire chemical vapor deposition”, Journal of Applied Physics 92: 4695-4698, 2002), or spray pyrolysis (Itoh, Abdullah, and Okuyama, “Direct preparation of nonagglomerated indium tin oxide nanoparticles using various spray pyrolysis methods, Journal of Materials Research 19: 1077-1086, 2004). These references, however, do not appear to disclose methods of forming a film or materials containing a mixture of compositionally and/or functionally distinct nanoparticles.
Although compositionally and/or functionally distinct particles may be mixed in pastes or inks, the particles tend to agglomerate in suspension due to the high particle concentrations required for printing and strong inter-particle forces. Even if a surfactant were used, agglomeration would continue after deposition as any surfactant would be lost during the drying and/or sintering process. This in turn limits the individual grain size of a component in a multicomponent film to the micron scale, which is acceptable for forming films of particles that are not compositionally different. Examples of methods that produce films made from a single composition particle are disclosed in patent applications 20070169812 and 20070169813, in which an ink is formed from an organic solvent and microflakes or nanoflakes produced by milling a solid having a predetermined mixture of elements.
Patterned nanoparticle films can be made in colloidal solutions using self-assembly techniques (see, for example, Sastry, Gole, and Sainkar, “Formation of patterned, heterocolloidal nanoparticle thin films”, Langmuir 16: 3553-3556, 2000). Here, the driving force is the electrostatic interactions between like or unlike particles and molecules. Although multiple particle types may be mixed, these techniques are generally very slow and are thus not suitable for continuous or large-scale fabrication of nanoparticle films.
Accordingly, a need exists for multicomponent materials, such as composites composed of distinct nanoparticles, and processes for their manufacture. There is also a need for such materials and processes to feasibly mass produce certain types of fuel cells, solar cells, batteries and other devices that can utilize multicomponent nanoparticle materials and films.